<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div class="box">
        <ul>
            <li>我是1</li>
            <li>我是2</li>
            <li>我是3</li>
            <li>我是4</li>
            <li>我是5</li>
        </ul>
        <button>点击</button>
    </div>

    <script>
        let uu = document.querySelector("ul");

        uu.onclick = (e) => {
            e = e || window.event;
            console.log(e.target.innerHTML)
        }
        let btn = document.querySelector("button");

        btn.onclick = () => {
            let li = document.createElement("li");
            li.innerHTML = "new"
            uu.appendChild(li)
        }
    </script>
</body>

</html>